Wear mechanisms
Adhesive wear
Material transfers between surfaces when microscopic contact points adhere and are sheared apart.
Abrasive wear
A harder surface or particle removes material from a softer surface.
Third-body wear
Particles such as cement, metal or bone enter the bearing and increase scratching and wear.
Fatigue wear
Repeated subsurface stress produces cracks, delamination or pitting.
Adhesive wear occurs when microscopic contact points bond and material is pulled from one surface by the other.
Abrasive wear occurs when a harder surface or particle scratches a softer surface. Third-body particles such as cement or metal debris can accelerate this process.
Fatigue wear results from repeated subsurface loading and crack propagation.
Corrosive or tribocorrosive processes combine mechanical disruption with electrochemical degradation, especially at modular metallic junctions.
Factors affecting wear
Wear depends on:
- bearing materials
- surface roughness
- lubrication
- contact stress
- component position
- head size
- activity
- third-body particles
- material processing and oxidation
Bearing couples
Common hip bearing combinations include:
- metal on polyethylene
- ceramic on polyethylene
- ceramic on ceramic
Each has a different balance of wear, fracture risk, noise, cost, head-size options and revision considerations.
Metal-on-polyethylene has a long clinical record and remains common. Ceramic heads can reduce scratching and are frequently paired with polyethylene.
Ceramic-on-ceramic offers extremely low wear but introduces different failure modes such as noise, edge loading, liner chipping or rare fracture.
Metal-on-metal bearings generate metal wear and corrosion products and can cause adverse local tissue reactions; this is a different biological problem from classic polyethylene particle disease.
Wear particles
Particles generated at an implant can be biologically active. Macrophages ingest particles and release mediators that promote osteoclast formation and bone resorption.
This pathway can lead to particle-associated osteolysis.
Osteolysis versus loosening
They are related but not identical.
Osteolysis
- focal or diffuse bone loss caused by biological response to debris
Aseptic loosening
- failure of implant fixation without infection
- may result from osteolysis, inadequate initial fixation, mechanical failure or a combination
An implant can show osteolysis before it becomes mechanically loose.
Osteolysis is bone loss caused by biological response to debris. A component can be well fixed despite surrounding osteolysis, particularly early. Loosening refers to loss of stable fixation and is assessed using symptoms, serial imaging and implant-specific radiographic criteria.
Tribology
Tribology is the study of friction, lubrication and wear.
In arthroplasty, good tribological performance requires:
- smooth bearing surfaces
- appropriate lubrication
- stable component position
- minimisation of third-body debris
Ceramic surfaces
Ceramics are harder and more scratch resistant than metal, which can help maintain a smooth bearing surface. Their main trade-off is brittle material behaviour.
Clinical principle
When evaluating an arthroplasty bearing problem, separate:
- mechanical wear
- biological response to debris
- loss of bone
- loss of implant fixation
Tribology in arthroplasty
Tribology describes friction, lubrication and wear between articulating surfaces. Bearing performance depends not only on the nominal material pair but also on:
- surface finish
- head size
- clearance
- component position
- lubrication regime
- edge loading
- third-body particles
- patient activity
- implant design
A low-friction bearing can still perform poorly if alignment or component position produces abnormal loading.
Polyethylene wear
Conventional polyethylene wear historically generated large numbers of small particles capable of driving macrophage-mediated osteolysis. Modern highly cross-linked polyethylene has substantially reduced wear in many applications, although oxidation, rim damage, impingement and mechanical failure remain possible.
Wear should be distinguished from creep. Early apparent penetration may partly reflect bedding-in or deformation rather than true removal of material.
Biological response to debris
Particles are phagocytosed by macrophages, which release inflammatory mediators that promote osteoclast formation and bone resorption. The result may be:
- focal osteolysis
- loss of implant support
- migration
- periprosthetic fracture
- eventual aseptic loosening
The process can be clinically silent for a long period. Surveillance therefore matters in patients with bearings or implants known to have higher wear risk.
Radiographic assessment
Review serial images for:
- progressive radiolucent lines
- focal cystic defects
- migration
- subsidence
- change in component position
- eccentric head position suggesting liner wear
- fracture of implant or cement mantle
Serial comparison is more informative than a single film.
FRCS synthesis
For a painful arthroplasty, do not assume wear simply because osteolysis is present. Exclude infection, establish whether components remain fixed, define the distribution of bone loss and identify the likely source of debris before planning revision.